The present invention relates to the formation of alumina impregnated carbon fiber mats. Such mats can be used, for example, in the sulfur electrode of a sodium sulfur cell.
In the discharge operation of a sodium-sulfur electrochemical cell, metallic sodium is oxidized at its interface with an ionically conductive membrane, usually of the beta-alumina family. The resulting sodium cations pass through the beta-alumina membrane and enter the sulfur electrode. The electrons generated from the oxidation pass from the sodium to an external circuit by means of a current collector and they return to the sulfur electrode of the cell by means of another collector. In this process, sulfur is reduced to polysulfide.
Typically, carbon felt or graphite is distributed throughout the sulfur compartment or electrode to provide electrical conductivity. Sodium sulfur cells of the type just described have exhibited several problems. The sodium ions, which during discharge first appear in the sulfur electrode compartment at the beta-alumina membrane interface, are not distributed rapidly enough throughout the sulfur electrode compartment. Further, during recharging, the sodium ions within the sulfur electrode compartment must move back toward the beta-alumina interface, pass back through the beta-alumina membrane and be reduced back to metallic sodium at the beta-alumina-liquid sodium interface. The rate of sodium ion transport within the sulfur electrode has limited the practical thickness of this electrode and hence the capacity of the cell.
The relatively slow movement of the sodium ions in the sulfur compartment also limits reactant utilization, due to the formation of isolated pockets of sulfur and/or sodium polysulfide. The presence of sulfur deposits, as well as variations in beta-alumina wall thicknesses and carbon mat non-uniformities, can affect local current densities. See, "The Sodium Sulfur Battery":, Edited by J. L. Sudworth and A. R. Tilley, 187-195, 1985, Chapman and Hall, London. Additionally, changes in the sulfur electrode reactant volume with state-of-charge can affect the sodium ion activity within the sulfur electrode compartment. The deleterious effects of this non-uniform behavior could be reduced if the movement of sodium ions were accelerated.
It is known that concentration polarization can be reduced in aqueous electrolytes by forced convection or stirring. This solution is impractical in sodium sulfur cells because of the porous nature of the sulfur electrode. Surface forces can be employed to aid transport through the carbon mat, by wicking or capillarity. It is also known that carbon surfaces are readily wet by elemental sulfur and not by polysulfides, whereas alumina surfaces are preferentially wet by sodium polysulfides. Therefore, it has been suggested that the use of a carbon fiber mat deposited with alumina should provide increased sodium polysulfide wicking and consequently improved transport within the sulfur electrode. See G. J. Janz and R. M. Murphy, Wettability of Some Carbon Surfaces by Molten Sulfur and Polysulfides, J. Electrochem Soc., 125, 1605 (1978) and G. J. Janz et al, Wetting Properties of the Sulfur Electrode in Sodium-Sulfur Batteries, J. Appl. Electrochem., 10, 789 (1980).
The formation of an insulating sulfur layer at the beta-alumina/sulfur electrode interface during recharging causes a large increase in cell resistance which shuts off the recharge reaction before all the sodium polysulfide can be converted to sulfur. This limits the reactant utilization. U.S. Pat. No. 4,084,041 issued to F. A. Ludwig discloses the use of oxide treated felt as a resistive layer adjacent to and contiguous with the cation permeable membrane, i.e., the beta-alumina tube. Alumina as a coating material is used to reduce wetting by sulfur and improve wetting by polysulfide. The larger objective was to permit, during recharging, oxidation of polysulfides to elemental sulfur rather than stopping the oxidation process at sodium pentasulfide, and in this way reactant utilization was improved. However, the use of an alumina-treated graphite felt was reported to yield an increased discharge cell resistance.
U.K. Patent No. 1528672 issued to Jones and Robinson discloses the use of carbon mats containing alumina, made by the process of mechanically mixing carbon fibers and alumina fibers together. Although high reactant utilization during prolonged cycling is achieved by this method, cell resistance is appreciably increased. See, for example, "The Sodium Sulfur Battery", edited by J. L. Sudworth and A. R. Tilley, pg. 187-195, Chapman and Hall, London (1985).
Thus, there is a need to reduce concentration polarization without increasing cell resistance in sodium-sulfur cells.